1. Field of the Invention
The present invention relates to an electrode system for optical modulation and an optical modulator.
2. Related Art Statement
A traveling-wave optical modulator using lithium niobate (LiNbO3), lithium tantalate (LiTaO3) or gallium-arsenide (GaAs) for the optical waveguide has excellent properties and may realize a broadband modulation at a high efficiency. Lithium niobate and lithium tantalate are excellent ferroelectric materials having large electrooptic coefficients and can control light within a short optical path. Factors suppressing the modulation frequency of the traveling-wave optical modulator include velocity mismatch, dispersion, electrode conductor loss and dielectric loss.
Such optical modulator has a relatively complex structure having a substrate, an optical waveguide, signal and ground electrodes, and a buffer layer. The dimensions of such elements have not necessarily been studied enough.
In a normal travelling wave type optical modulator made of lithium niobate has dimensions shown, for example, in FIG. 5. A pair of ground electrodes 3A, 3B and a signal electrode 14 are provided on a substrate 12 through a buffer layer 9. Optical waveguides 6A and 6B are provided between the ground electrodes 3A, 3B and signal electrode 14. For example, it is provided that the substrate 12 is made of lithium niobate single crystal and the thickness “T” of the substrate is made sufficiently large. In this case, it is possible to realize the velocity matching when the width “W” of the signal electrode is 8 μm, the gap “G” is 20 μm, the thickness “E” of the electrode is 18 μm and the thickness “B” of the buffer layer 9 made of SiO2 is 1.3 μm. The product (Vπ·L) of the driving voltage Vπ and the length “L” of the electrode is, however, elevated to 20 Vcm, so that the resultant optical modulator requires a high driving voltage. Further, the propagation loss in the electrode due to conductor loss is as large as 0.32 dB/cm at 1 GHz.
The assignee filed Japanese patent publications 10-133, 159A and 2002-169, 133A, and disclosed a travelling wave optical modulator for giving the solution. The modulator has an optical waveguide substrate having a thinner portion with a thickness of 10 μm or smaller where the optical waveguide is formed. It is thereby possible to realize high-speed modulation without forming a buffer layer made of silicon dioxide, and to advantageously reduce the product “Vπ·L” of a driving voltage Vπ and a length “L” of an electrode.